Security documents and valued goods are marked with materials exhibiting particular physical or chemical properties (security features), which serve to authenticate the marked items through a detection of the presence of said properties.
A common way to mark a security document or a valued good comprises the incorporation of one or more marking materials into a printing ink or a coating composition, which is subsequently applied to said document or good, if needed in the form of indicia. Alternative ways to mark an article comprise the incorporation of one or more marking materials into the bulk (plastic, paper, liquid, etc.) of the article.
Physical properties which have been used as a security feature include noteworthy selective spectral light absorption in the ultraviolet (UV), visible, and infrared (IR) wavelength range, as well as prompt and delayed UV-, visible-, and IR-luminescence, such as disclosed e.g. in U.S. Pat. Nos. 3,473,027; 3,412,245; 3,582,623; 3,663,813; 3,650,400; 3,566,120; 3,455,577; and 4,202,491. Security features allowing for a contact-less detection respectively reading of the marking, e.g. by optical means, were always perceived as a preferred option.
Markings exhibiting a viewing-angle dependent light reflection spectrum (“optically variable devices”, OVDs) are used as an efficient anti-copy means on bank notes and security documents. Among the OVDs, optically variable inks (OVI®; EP 227,423 B1) have acquired a preeminent position since their first introduction on currency back in 1987. Such inks are formulated on the basis of optically variable pigment (OVP), a preferred type of OVP being the flaky thin-film optical interference device described in U.S. Pat. Nos. 4,705,300; 4,705,356; 4,721,217; 4,779,898; 4,930,866; 5,084,351 and in related disclosures. Other useful types of OVP comprise the multiply coated particles described in U.S. Pat. Nos. 5,624,486 and 5,607,504.
Still another, useful type of optically variable pigments is obtained through the photo-polymerization of a thin film of cholesteric (i.e. chiral-nematic) liquid crystal material, followed by comminuting the polymer film into a pigment, such as described in U.S. Pat. Nos. 5,807,497 and 5,824,733. Said liquid crystal based (LC-) pigments have the additional property of reflecting, depending upon their internal chirality, selectively either right-handed or left-handed circularly polarized light, as disclosed in EP 899,119 B1. As the LC-pigments can be made in either, right- or left-handed chirality, the circular polarization sense of the reflected light from LC-pigment can be exploited to impart an additional, covert security feature to a document or article.
Still another type of optically variable devices can be realized on the basis of diffraction gratings, e.g. in the form of embossed holograms or similar devices carried on a metallized polymer foil which is applied to a document or an article. Said embossed polymer foil can also be comminuted into a pigment and used as ‘glitter’ in a coating composition. In still another, somewhat less efficient way, a diffraction grating is embossed into preformed, pigment-size metal (aluminum) flakes. In all these embodiments, the required diffraction grating structure has a line spacing comparable with the wavelength of the diffracted light, i.e. typically of the order of 300-500 nanometers, corresponding to 2,000 or more lines per mm.
Optically variable pigments, inks and printed features, as well as optical diffraction devices, can be identified by assessing their spectral reflection properties for at least two different angles of view. Such information is commonly obtained in the laboratory with the help of a gonio-spectrometer (e.g. as manufactured by Zeiss), as described by R. Maisch and M. Weigand in “Perlglanzpigmente”, 2nd edition, Die Bibliothek der Technik, Vol 56, Verlag Moderne Industrie AG, Landsberg/Lech, 1992 and in the cited references therein. A gonio-spectrometer allows noteworthy to study a sample under any combination of illumination angle and spectral analysis angle.
In preferred technology, the detector cost is kept low by avoiding the use of spectrometers and adopting the sequential color-LED illumination technique disclosed in U.S. Pat. No. 4,204,765. This document describes a device for testing colored securities, such as paper having colored areas imprinted thereupon. A plurality of light emitting diodes (LEDs), each emitting light of a different wavelength range, sequentially illuminate a determined area on said paper, which is more or less reflective for the incident light. A single photodetector receives the light reflected by the paper and delivers an electric signal corresponding to the received light intensity. By comparison of the measured signals for the various LEDs with predefined reference values, an indicator of the authenticity of said paper is derived.
The technology disclosed in U.S. Pat. No. 4,204,765, although it is very compliant with a low-cost device, is not suitable for the testing of optically variable security features due to its single observation-angle design.
A system for the automated verification of optically variable features on value documents, bank notes, etc. has been disclosed in WO 01/54077. According to this disclosure, the optically variable feature is illuminated at least with a first and with a second light beam, and the light reflected from it is analyzed for at least a first and a second observation angle. The automated verification technology disclosed in WO 01/54077 has, however, a number of shortcomings which prevent in particular its practical implementation in low-cost automatic reader devices.
A first shortcoming of the technology of WO 01/54077 is tied to the measurement geometry. The disclosed device illuminates the optically variable feature at predetermined incidence angles using directional light beams, and spectrally analyzes the light reflected from the illuminated feature at predetermined reflection angles related to said incidence angles. For optically variable ink, the incident and reflected light beams roughly obey the mirror law, i.e. the incidence and reflection angles are about opposed-equal. This is not the case for optically variable devices based on holographic diffraction gratings, where incidence and reflection angle can be generally different from each other. By fixing both, incidence and reflection angle in the hardware lay-out, the system of WO 01/54077 can thus only be used for the authentication of the very determined type of OVDs for which it was conceived.
A second shortcoming of the technology of WO 01/54077 is in the expensive nature of the components used. Two or more spectrometers are noteworthy required to analyze the light collected at two or more different angles of reflection. The cost of an authentication device containing spectrometers is likely to prevent its use in an automatic vending machine for low-cost products.
A third major shortcoming of the technology of WO 01/54077 lies finally in its lack of miniaturize-ability. Noteworthy, the analysis of spectral reflectance data at near-grazing incidence is likely to result in a physically extended measurement set-up. This in turn is a rather prohibitive constraint for applications in automated vending machines, where severe space restrictions must be obeyed.